The present invention generally relates to an improved collision filter and more particularly to such a filter for use in a tap transceiver of an Ethernet-type LAN network.
Ethernet-type LAN networks are well known in the art. Such networks employ Manchester encoded data packets for conveying information from one node of the network to another node. Each node of the network generally includes a tap transceiver. The main function of a tap transceiver is to perform signal translation to the encoded signals and to detect a fault condition on the network which results when two or more nodes are simultaneously transmitting on the network. The last noted function is performed in the tap transceiver by a collision filter.
The Manchester encoded signals are digital signals with two discreet voltage levels, for example, zero (0) volts and minus two (-2) volts. Hence, when a node is transmitting, there is an average DC voltage level on the network of about minus one (-1) volt. When two nodes are simultaneously transmitting, the discreet signal levels can vary from zero (0) to minus four (-4) volts, resulting in an increase in the magnitude of the average DC voltage level on the network to about minus two (-2) volts. In order to detect a fault condition, it is therefore necessary to extract the average DC voltage level on the network while rejecting the AC components of the encoded signals.
To perform the required signal processing for detecting fault conditions, collision filters have taken the form of cascaded low pass filters. One known collision filter has employed three such cascaded low pass filters. These filters, by virtue of their attenuation characteristics, pass the DC levels while rejecting the AC components.
While the prior art cascaded low pass filters have been generally successful, there remains room for improvement in their operation. One problem has been that, due to the monotonic attenuation characteristics of each filter stage and hence, the monotonic characteristic of the overall filter, in order to attenuate the lower fundamental frequency of five (5) megahertz adequately, the higher frequency components, including the higher fundamental frequency component of ten (10) megahertz, have been overattenuated to the end of unduly decreasing the transient step response times of the filters.
Tap transceivers, including the collision filters, are generally implemented in integrated circuit form. When implemented in such form, it is necessary to take into account the process variations of the resistors and capacitors used in the prior art low pass filters. This has resulted in the overdesign of such filters to assure adequate attenuation of the five (5) megahertz lower fundamental frequency. This unfortunately has resulted in greater attenuation of the higher frequencies, including the higher fundamental frequency, than necessary, and hence, slower than optimum transient response times.
The present invention overcomes these design constraints by utilizing a single low pass filter stage followed by one or more notch filter stages. This results in adequate attenuation of the five (5) megahertz frequency component along with adequate, but not over attenuation, of the higher frequencies. What results is a collision filter having adequate attenuation for extracting the average DC voltage level on the network while also having improved transient response times.